Calculations - GE NBVllA Series Instructions Manual

VOLTS
120
120
208
In order to demonstrate how the V R pickup voltage,
is related to the negative phase-sequence voltage,
CHARACTERISTICS,
show how the negative phase sequence is computed.
Where
N egative phase-sequence voltage
V N
=
1 / 120
a
=
2
1 /240
a
It should be noted that in this analysis V N is based on line-to-line voltages and so
will be greater by the square root of
calculated from phase-to-neutral quantities.
When we apply a single-phase voltage to the relay,
resultant voltages seen by the relay are the same as the relay would see if it were
connected to a three-phase system that had one phase shorted.
is shorted, then in equation
v12
0
V 1-2
=
- v2-3
v2-3
=
( 1)
Then equation will reduce to:
v2-3
=
At Pickup:
J3
V R
=
After the amount of unbalance that can be tolerated in the line voltage (V N ) has
been determined, then the pickup voltage can be readily obtained from equation
For the two types of unbalance shown in Figure
is the same in terms of negative-sequence voltage V N , but not in
shift, Vs. The
determined from the voltage unbalance that it is desired to detect.
1
Consider an unbalance voltage that results from a shift of the apex of the voltage
GEI-50261
FREQUE N CY
60
60
60

CALCULATIONS

( 1)
V 2-3
V N since V R
=
following examples
TABLE IV
CURRE N T (AMPS)
0.1 14
0.995
0.056
as defined 1-n the section on
3
than the negative-sequence voltages as
as shown in Figure
in this case.
9,
the sensitivity
illustrate how
6
VOLT - AMPS
13.68
1 1.40
11.60
we will first
(1)
Thus, if we assume
(2)
of the N BV relay
terms of voltage
the relay
setting
10,
the
2.
may be